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There's been a lot of hubbub recently over mobile carriers purposefully slowing down data speeds for customers still on unlimited plans.

It's nothing new, at least for the last year or so. But what is kind of new, confusing, and insanely frustrating is how and when carriers decide to slow down (or "throttle") your data speeds.

AT&T tried to clear things up last week by saying it will start throttling data speeds once an unlimited data customer uses 3 GB of data in a billing cycle. That's the same cap set for customers on a tiered plan. Customers with one of the newer 4G LTE phones get a better deal. Their data speeds will be throttled once they reach 5 GB in a billing cycle. That's because AT&T's LTE network is relatively small right now and isn't as crunched for bandwidth like its 3G network is.

So AT&T has effectively killed its unlimited plan by making it as close to the tiered plan as possible.

But AT&T isn't alone. T-Mobile and Verizon also throttle their unlimited users, but are less clear about the threshold you have to hit before they do so or how much they slow you down.

Sprint is the only major carrier that still offers unlimited data without throttling. It's also the only carrier that sells the iPhone and offers full speed unlimited data.

It's all very frustrating. And confusing.

So let's make it easy for you.

Here's the data plan breakdown for all four major carriers:

Verizon

New customers can get 2 GB for $30 per month, 5 GB for $50 per month, and 10 GB for $80 per month. If you go over those limits, Verizon charges you an extra $10 and gives you another gigabyte of data to use in the billing cycle.

If you're grandfathered into an unlimited data plan with Verizon, the carrier will start throttling your device if you're in the top 5 percent of data users for that month. What that really means is anyone's guess.

AT&T

New customers must get a tiered data plan. You get 300 MB for $20 per month, 3 GB for $30 per month, or 5 GB for $50 per month. If you have one of the latter two plans, you'll be charged $10 and given another gigabyte of data if you go over your limit. AT&T will charge you an extra $20 for another 300 MB if you go over with the first plan.

For those of you still on an unlimited plan, AT&T will start throttling your data speeds once you use up 3 GB of data in one billing cycle. If you have a 4G LTE phone, you can use 5 GB of data before AT&T throttles your speeds.

T-Mobile

T-Mobile has three so-called "unlimited" plans, but each one has a different threshold for when the carrier will start throttling your data speed. T-Mobile bundles text, voice, and data into one flat-rate package, which is why these prices may look more expensive than AT&T and Verizon's data plans. In fact, T-Mobile's plans tend to be cheaper overall.

Here's what you get with unlimited text and voice factored in: For $79.99 per month T-Mobile will start throttling your speeds after 2 GB of data usage. $89.99 per month gets you 5 GB of data before throttling kicks in. Finally, $119.99 per month will get you 10 GB of data usage before T-Mobile slows down your connection.

Sprint

Sprint also bundles its data plans with voice and text messaging. You can get unlimited data and 450 minutes talk time for $69.99 per month. It goes up from there, with the unlimited everything plan costing $99.99 per month.

Sprint will not throttle your data speeds, no matter how much you use per billing cycle.

What We Still Don't Know

The big question is: How much do carriers slow down your connection after you reach their threshold? We can only go by anecdotal evidence. There are some videoslike this one that give you a side-by-side comparison of data speeds on a throttled versus non-throttled phone. In many cases, the throttled phones appear to be painfully slow.